MINERALOGICAL MAGAZINE

VOLUME 61 NUMBER 409 DECEMBER 1997

The Minastira peraluminous , Puno, southeastern Peru: a quenched, hypabyssal intrusion recording magma commingling and mixing

DANIEL J. KONTAK1 AND ALAN H. CLARK Department of Geological Sciences, Queen's University, Kingston, Ontario, Canada K7L 3N6

Abstract The Minastira granite, a c. 25 Ma subvolcanic plug of fine-grained granitic rock in the Cordillera Oriental of SE Peru, has preserved textures indicative of a history involving mixing of at least two magmas, a volumetrically dominant felsic component and a less voluminous marie one. The felsic component is represented by variably fractured, altered and embayed crystals of quartz, feldspar, biotite with minor coarse- grained melt- and fluid-inclusion rich apatite, and possible cordierite (now a pseudomorphous Fe-Mg phase), whereas the marie component is represented by calcic plagioclase. The process of magma mixing is reflected by: (1) ubiquitous sieved-textured plagioclase with complex textural relationships; (2) a large range in plagioclase compositions with reversals and spike patterns in profiles; (3) embayed and internally fractured (thermal shock?) quartz; (4) the rare occurrence ofpyroxene coronas on quartz; and (5) textures within biotite suggestive of its incipient breakdown. The lack of marie enclaves indicates that physico-chemical conditions of the mixing were conducive to homogenization (i.e. chemical diffusion) and a superficially homogeneous rock is now observed. The association of glomeroclasts of crystals originating from both the marie and felsic end members and a quenched quartz-feldspar matrix indicate that the mixing occurred in an underlying magma chamber. KEYWORDS: granite, peraluminous, magma mixing, Minastira, Peru.

Introduction Pliocene age that record the involvement of both mantle and crustal reservoirs (Kontak et al., 1984, THE Cordillera de Carabaya segment of the Cordillera 1985, 1990; Clark et al., 1984, 1990a; Sandeman et Oriental of SE Peru (Fig. 1) exposes a range of aL, 1995). In several districts, field, petrographic and intrusive and extrusive magmatic suites of Permian to petrochemical relationships strongly suggest that the influx of mantle-derived basic magma contributed to the fusion of the continental crust under conditions in which the 'parental' and 'daughter' magmas could Present address: Nova Scotia Department of Natural subsequently commingle at shallow depths. For Resources, P.O. Box 698, Halifax, Nova Scotia, Canada example, Kontak et al. (1986) demonstrated that the B3J 2T9. Mineralogical Magazine, December 1997, Vol. 61, pp. 743-764 Copyright the Mineralogical Society 744 D.J. KONTAK AND A. H. CLARK

I I /~' 72 70030 ' \1 70 ~ // 200 km 0 San Gaban % , %, 0

~"~B olivia

14 ~

Limbani Pluton

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~ Jurassic Macusani Syenite 25 km Limacpampa ~ Triassic San Gaban Complex and ~ Permian Mitu Group Related Intrusions

FIG. 1. Location of the study-area (Cordillera de Carabaya) in the Central Andes of Peru with the broad lithotectonic subdivisions shown. Simplified geology shows the distribution of igneous rocks, intrusive and extrusive, in the study area with the area in white mainly underlain by a dominantly Lower Palaeozoic package of metasedimentary rocks. Location of the Minastira granite is indicated by the arrow (note that the size of the intrusion is too small to show at this scale). c. 25 Ma Cerro Moro Moroni (or Moromoroni) the contribution of the felsic melt component to the volcanic suite exhibits both mineralogical and exposed extrusive rocks was thus minor. chemical evidence for commingling and mixing of Subsequently, work by Sandeman et al. (1996a, b) K-rich marie (absarokitic) and highly peraluminous has shown that this commingling of mafic and felsic felsic melts, the latter represented by a cross-cutting melts is widespread throughout the Cordillera de biotite monzogranite dyke. However, the basic Carabya region of SE Peru, and involved both basaltic character of the volcanic suite was maintained and and lamprophyric marie end-members. PERALUMINOUS GRANITE FROM PERU 745

Several broadly coeval peraluminous cordierite (__+ plugs (<100-1000 m 2 in area) hosted by red quartz sillimanite)-biotite monzogranite stocks have been arenites of the Cretaceous Cotacucho Group delimited within a few km of the Moro Moroni centre (Laubacher, 1978). The contacts are not exposed (Clark et al., 1983, 1984 Kontak et al., 1987; Kontak and no thermal aureole is detectable. Phenocrysts of and Clark, 1988; Sandeman, 1995), but the role of quartz, translucent alkali feldspar and biotite are magma mixing processes in their evolution is unclear. evenly distributed in an aphanitic, dark-green matrix. However, a small hypabyssal body of f'me-grained The intrusive rocks appear to be unweathered. granitoid rock in the Minastira district (Fig. 1) is Conventional K-Ar dating of unaltered biotite interpreted herein to represent the product of a system (Kontak et al., 1987) yields an age of 24.2 __+ 0.5 of magma mixing where the felsic component was Ma, indicating that intrusion occurred in the latest- dominant. The subvolcanic setting of this intrusion has Oligocene, coeval with eruption of the marie-to- permitted the preservation of petrographic evidence of felsic Picotani Group strata of the Cordillera de the mechanisms of magma commingling and mixing Carabaya (Sandeman et al., 1996a, b). which would normally not survive either the protracted annealing unavoidable in deeper-seated Petrography of the Minastira granite plutons or the violence of pyroclastic eruption. Xenolith Geological setting A single inclusion (2 mm) of metasedimentary rock The study area is located within the Inner Arc system consists of fibrolitic sillimanite, and lesser biotite, (Clark et aL, 1984, 1990a) of the Central Andean plagioclase, spinel and zircon. The absence of a orogen, broadly cospatial with the Cordillera Oriental reaction rim about the fragment suggests that it was of Peru and Bolivia (Fig. 1). In this region, post- entrained as the magma approached its level of Carboniferous magmatism has been both episodic and emplacement. Similar xenoliths have been reported extremely varied in nature (Kontak et aL, 1984, 1990), by Injoque et al. (1983) to occur in an Upper contrasting with the quasicontinuous and largely talc- Miocene subvolcanic intrusion in the Choquene alkaline subduction zone-related magrnatism of the mining district, c. 80 km ESE of Minastira Main Arc (e.g. Pitcher et al., 1985, and references (Yamamura, 1991). therein). The Inner Arc is located at the interface between the Brazilian craton and the Andean orogen Glomeroclasts (Kontak, 1985), and the magrnatic evolution of this narrow lithotectonic domain has been strongly Gtomeroclasts, of 2-3 mm size and exhibiting influenced by the gross tectonic relationships granophyric texture, are observable only in thin between the two geotectonic provinces. section. Two types are distinguished: (1) quartz- The geological evolution of the Cordillera de alkali feldspar-biotite-brown ferromagnesian phase Carabaya is reviewed elsewhere (Laubacher, 1978; (Fig. 2a); and (2)) sieve-textured plagioclase-biotite Kontak, 1985). The oldest-exposed rocks, pelitic and ___ hercynitic spinel __+ brown ferromagnesian phase psammitic strata of - age which ___ apatite (Fig. 2b). These mineral phases also occur attain 15 km in thickness, are overlain unconform- as individual grains in the granite (see below), but ably by some 3 km of Carboniferous-Lower Permian this is the only observed occurrence of spinel of psammites and limestones and up to 3 km of probable magmatic origin in the Minastira rocks. It is Permian molasse sediments. Magmatism in the emerald-green, fine-grained, euhedral, and occurs as region commenced in the Early Permian (Clark et inclusions within plagioclase grains adjacent to the al., 1990b), prior to the initiation of Andean brown ferromagnesian phase. The second type of orogenesis in the mid-Triassic, and persisted to at glomeroclast is distinguished by a granophyric least the Pliocene. The most voluminous igneous texture (Fig. 2a) and arrays of negative-shaped, rocks are those of the Upper Triassic-Lower Jurassic vapour-rich inclusions in quartz. Carabaya Batholith (Kontak et al., 1985, 1990), which displays evidence of commingling and mixing Mineralogy of the granite of melts derived from the mantle and crust. The much younger rocks described herein are representative of The intrusive rocks exhibit a mineral assemblage the San Rafael intrusive suite, a lithodemic unit of the with several unusual, and in part problematic, Upper Oligocene-Lower Miocene Picotani Group, features, as summarized in Table 1 and illustrated itself part of the~Crucero Supergroup (Sandeman, in Figs. 2 and 3. 1995; Sandeman et al., 1996a, b). Quartz phenocrysts (< 1 mm to 5 ram) are either The Minastira granite (latitude, 14~ .... S; euhedral or embayed, the latter grains transected by longitude 70~ '' W) comprises several small fractures disposed in curving arrays paralleling their 746 D.J. KONTAK AND A. H. CLARK PERALUMINOUS GRANITE FROM PERU 747

margins. The fractures are filled with t'me-grained, traces. This latter texture is similar to that described microscopically brown and grey material that is by Nixon (1988) and Stimac and Pearce (1992), and apparently identical to the matrix phases of the rock. attributed to the breakdown of biotite due to Narrow brownish, crystalline rims surround some dehydration above its stability field. A reddish and quartz grains and are probably pyroxene (cf. Sato, transparent phase is tentatively identified as rutile. 1975; Kontak et al., 1986; Stimac and Pearce, 1992). Biotite is generally free of other inclusions, but some The quartz phenocrysts host glass-bearing and fluid grains contain: (I) coarse apatite crystals, some inclusions, bodies of a brown, apparently secondary projecting into the matrix (rare); (2) plagioclase laths alteration phase with or without a feldspar rim, zircon (very rare); and (3) bodies of an unidentified brown euhedra lacking corroded cores, and flakes of biotite. phase that appears similar to that occurring in the The glass-bearing inclusions are diverse, and include matrix of the rock (see below). Biotite rarely occurs examples: (1) with rectangular outlines, containing as inclusions in plagioclase, where it is finer-grained microlites of unknown composition (feldspar?) in an but again contains the aforementioned opaque and isotropic medium, and with narrow border-zones of a related phases. Where several biotite grains are crystalline material with undulose extinction and hosted by a single plagioclase crystal, they are incorporating a fine-grained opaque phase; (2) with concentrically arranged about its core. rectangular shapes but consisting of glass, a vapour Plagioclase (< 1 mm to 6-8 mm) occurs both as bubble and a microlite; (3) comprising green and single grains of variable habit (Fig. 2c, d,e) and as a brown birefringent phases + glass + an opaque constituent of glomeroclasts (Fig. 2a, b). The coarser mineral __. a vapour bubble; and (4) containing an grains are generally euhedral-to-subhedral but some opaque phase and glass in variable proportions. are broken; in contrast, the finer grains are euhedral. Other, probably allied inclusions are made up of In detail, the plagioclase is very heterogeneous in completely crystallized, feather-textured and in part form, including: (1) simple grains that are normally spherulitic material of feldspathic composition zoned and contain few inclusions (Fig. 2c); (2) (qualitative electron microprobe data) and minor euhedral, zoned grains exhibiting varying amounts amounts of equant opaque minerals. Semi-quantita- of a brown ferromagnesian phase (Fig. 2e; up to 90% tive electron-probe microanalysis of several opaque of the finer-grained plagioclase may be replaced by phases in inclusions of types 1 and 3 reveals the this phase: see below) (3) coarse grains with presence of both an Fe-Ti oxide mineral containing pervasive or, more widely, marginal sieve texture minor Cr (< 1 wt.% Cr203) and a Cu-Fe sulphide with (Fig. 2d); (4) rare grains with zonally-concentrated optical properties and approximate composition inclusions of sillimanite, biotite, zircon and the conforming to those of chalcopyrite. brown ferromaguesian phase (Fig. 2e); and (5) grains Biotite (2-3 mm) occurs as brown micropheno- with 30-50 pm, vapour-dominated, fluid inclusions trysts with variable absorption (Fig. 2b, e) that are of very irregular shape (aspect ratios, 25-50:1). incipiently altered to chlorite and invariably contain Sanidine, a minor constituent, forms 2-9 mm numerous fine-grained inclusions of ilmenite and of grains of euhedral-to-anhedral habit, most commonly unknown, probably secondary, phases occurring microphenocrystic, and some with biotite, glass and either at the margins of the grains or along cleavage vapour-rich fluid inclusions. The grains are either

Fro. 2. Transmitted-light photomicrographs of the Minastira granite. All photos taken in crossed nicols except for e, g and h. (a) Glomeroclast showing granophyric texture. Note that the feldspars are unaltered and do not exhibit sieve textures. Width of field is 1 cm. (b) Glomeroclast consisting of plagioclase and biotite. The plagioclase has sieve textures, is cored by a brown ferromagnesian phase (see text), and is strongly zoned. Width of field is 1 cm. (c) Euhedral, strongly-zoned plagioclase grain clear of both mineral and melt inclusions, and alteration phases. Width of photo is 6 mm. (d) Strongly-zoned, subhedral plagioclase grain with sieve-textured core; note that the overgrowth border is free of melt inclusions. Width of photo is 1.3 mm. (e) Euhedral plagioclase with oriented sillimanite grains outlining the core and a zone near the grain margin containing the brown ferromagnesian phase. The grain in the upper right is partially resorbed quartz and the dark, acicular grains in the matrix are biotite. Width of photo is 5 mm. (t) Alkali feldspar grain showing incipient stages of conversion to unidentified material with perlitie-like texture (possibly internal melt) along fractures traversing the grain. This is the only grain in which this phenomenon was observed. Width of photo is 5 mm. (g and h) Subhedral to euhedral grains of a brown to greenish-brown ferromagnesian phase illustrating some of the unusual textural relationships. In Fig. 2g a plagioclase mantle is seen surrounding two grains of the brown phase (centre and lower right of photo) whereas in Fig. 2h variations in eolour occur within the alteration phase (not seen in black and white) and additional phases are locally developed (core area of grain). Width of photos are 0.67 mm (g) and 1.3 mm (h). 748 D.J. KONTAK AND A. H. CLARK

"~-'0 ~~ ~.~ .o =s

~ ~.~.~

0

0 o~ ~

o

0

R

=~ .~ o =~ v N o 0

R~ ~oD

E

o~ 0 PERALUMINOUS GRANITE FROM PERU 749

FIG. 3. Back-scattered electron (BSE) images and photomicrographs of matrix material in the Minastira granite. (a) and (b) are BSE images illustrating the two components of the matrix. The dark material is silica-rich (i.e. 94-99 wt.% SiO2) whereas the pale is enriched in silica, alkalis and alumina. Note that Fig. 3b is an enlargement of the boxed area in Fig. 3a. Points 1 and 2 in Fig. 3b refer to analyses given in Table 3. (c) X-ray image for the area shown in Fig. 3b illustrating the distribution of K which essentially matches the paler areas. The areas that are depleted in K correspond to the silica-rich areas in Fig. 3b (i.e. dark patches). Scale bar is 100 Ixm. (d) Photomicrograph (crossed nieols) of the matrix illustrating the generally hypidiomorphie granular texture (long dimension about 2.6 mm) (e) Photomicrograph (crossed nicols) of the malrix showing its typical texture (long dimension about 0.67 mm.). 750 D.J. KONTAK AND A. H. CLARK embayed or rimmed by fine-grained, unidentified, Geochemistry reddish-brown or colourless phases. Some coarse Analytical procedures sanidine grains have ragged interiors with volumes of perlitic-textured material (Fig. 2/). Analyses of whole-rock samples of the Minastira Apatite, an abundant constituent, occurs as coarse, granite and a biotite separate have been determined generally euhedral grains in the matrix and more (Kontak, 1985) using a Siemens Model VRS X-ray rarely in biotite and plagioclase phenocrysts. Fine, Fluorescence Spectrometer on fused (major dark lamellae that define crystallographic planes, and elements) and unfused (trace elements: Rb, Sr, Ba, reminiscent of 'sagenite', are commonly developed Nb, V, Cr, Zr, Y, Sn) powders. Additional trace in the margins of the apatite grains and in some cases elements (Cu, Pb, Zn, Ni) were analysed on an extend into their cores. Other apatites contain Instrumentation Laboratory Model 251 Atomic numerous euhedral mineral inclusions, crudely Absorption Spectrophotometer, while ferrous iron defining concentric patterns and including zircon, contents were determined by titration. A single monazite, rare sillimanite and at least three unknown whole-rock sample was analysed for REE using the phases. Some apatite grains enclose a variety of melt- thin-film X-ray fluorescence method of Fryer (1977). derived inclusions, including: (1) undevitrified glass- The compositions of biotite, feldspars, apatite and vapour-crystal systems, in which the crystals (1-3 oxides, as well as of the unknown alteration (?) phases) include an opaque phase and two birefringent phases and the rock matrix, were determined using a minerals, one equant and the other acicular; (2) JEOL 733 Superprobe, using a combined wave- bodies of feldspathic material with a 'spinifex-like' length- and energy-dispersive analytical technique texture similar to that observed in inclusions in quartz and the following operating conditions: accelerating (see above), together with grains of a red, probably voltage 15 kV, a probe current of about 5 nA and a Fe-bearing, mineral; and (3) undevitrified glass beam diameter of about 1-2 gtm (defocussed to lacking crystalline phases. 5 ~un for feldspar and matrix). Data reduction was One of the most common phenocrystic components carried out with the ZAF soft-ware program. is a brown to greenish-brown ferromagnesian phase exhibiting a variety of textures. It displays rectangular, Whole-rock chemistry cubic, pentagonal and hexagonal sections, varies in size from microlitic to phenocrystic (4-5 mm), ranges Analyses of two granite samples are recorded in from brown to olive-green, some grains being colour- Table 2 and are compared to analyses of the coeval, zoned, and is commonly mantled by a euhedral rim of strongly peraluminous, cordierite-bearing San Rafael zoned plagioelase (Fig. 2e,g,h). In high magnification granite (Palma, 1981; Kontak, 1985). The two radial, cuspate and colloform textures similar to clay samples of the Minastira granite are similar in minerals are observed. This phase also occurs coting composition, with the exception of TiO2 and a plagioclase, within biotite and rarely in apatite. granodioritic composition is defined (66 wt.% SiO2). Although no single mineral can be reconciled with The rock is markedly peraluminous (A/CNK=1.56 the range of morphology, some grains resemble and 1.87), strongly reduced (FeEO3/FeO = 0.83 and pinnitized cordierite. 1.16) and K-rich (KEO/Na20 = 1.55 and 1.86); the The matrix of the rock consists of rnicrolites of cafemic components, MgO, CaO, MnO and TiO2 are most of the above mentioned constituents embedded depleted. The P205 content is similar to that of the in an equigranular quartz-alkali feldspar intergrowth San Rafael granite and other peraluminous granites (Fig. 3) which, in high magnification back-scattered of similar SiO2 content (e.g. London, 1992; electron (BSE) imaging, reveals crudely feathery Pichavant et al., 1992). textures (Fig. 3a, b). The latter textures are very Trace element contents for the two samples (avg. in similar to those figured in London et al. (1993) and ppm) are as follows: Rb (341), Ba (725), Sr (153), Zr generated through devitrification of H20-saturated, (150), Sn (13), Nb (12), V (58), Ni (13), Cu (12), Pb subaluminous haplogranite glasses. (56) and Zn (83); these abundances compare well with In summary, the petrographic features of the those in the average S-type granite of the Lachlan Fold Minastira granite clearly record an association of Belt of Australia (White and Chappell, 1983). phases which are not entirely in mutual equilibrium Compared to the San Rafael granite, the Minastira and have either individually or collectively under- intrusion is enriched in Nb, depleted in Ba, Sr and Zr, gone a complex history. The presence of such comparable with respect to V, Rb and Sn (Table 2), features as embayed and corroded quartz grains and in terms of element ratios has higher Rb/Sr, lower with pyroxene rims, sieve-textured plagioclase, and K/Rb and comparable Ba/Sr. There is no enrichment embayed sanidine with internal melting features is, in metalliferous elements such as Cu, Pb, Zn or Sn. by analogy to other petrological settings, strongly The chondrite-normalized REE pattern for the suggestive of magma mixing. granite is compared in Fig. 4 to the field (n = I5) PERALUMINOUS GRANITE FROM PERU 751

TABLE 2. Whole-rock geochemistry for Oligocene-Miocene granites, SE Peru

Material Granite Granite Biotite Granite Granite Sample CC-282 CC-283 CC-284 CC-406 CC-408

SiO2 66.91 66.42 35.2 66.4 66.5 TiO2 0.14 0.43 3.69 0.48 0.49 A1203 16.2 15.33 16.98 16.1 15.91 Fe203 1.18 1.38 1. I 2.62 3.27 FeO 1.42 1.18 17.5 NA NA MnO 0.03 0.04 0.11 0.07 0.1 MgO 1.48 1.79 10.84 1.55 1.44 CaO 1.49 1.07 0.33 1.34 0.78 Na20 3.03 2.58 0.62 3.41 2.71 K20 4.72 4.82 8.52 4.59 5.64 P205 0.21 0.23 0.12 0.21 0.21

Trace elements (ppm) Rb 338 344 787 310 358 Sr 176 130 16 294 209 Ba 701 751 1868 1194 911 Nb 13 12 79 7 8 Zr 155 145 194 186 187 V 62 54 398 63 57 Cr NA NA 372 48 48 Ni 14 11 NA NA NA Cu 11 12 NA NA NA Pb 63 49 NA NA NA Zn 87 80 NA NA NA Sn 11 16 NA 20 4

Rb/Sr 1.92 2.64 - 0.05 1.71 Ba/Sr 3.98 5.77 - 4.06 4.35 K/Rb 116 116 - 122 130 A/CNK 1.56 1.87 - 1.24 1.33

Samples CC-282, 283 and 284 from Minastira; Samples CC-406 and 408 from San Rafael

delimited by other peraluminous Oligocene-Mioeene overall for all samples and are typical ofperaluminous granitoids of SE Peru, including a single analysis of an suites (e.g. Muecke and Clarke, 1981). exceptionally fresh granodiorite (CC-406) from San Rafael. The latter sample is from the upper, chilled Matrix composition part of the San Rafael intrusion (Kontak and Clark, 1988) and is considered to be representative of an The bulk composition of the matrix of the Minastira early and relatively undifferentiated facies of that granite may represent the magma in equilibrium with pluton. The REE pattern for the Minastira granite is the phenocryst assemblage (i.e. rim phases). Because strongly fractionated, with (La/Yb)N = 69 and a strong of its very fine-grained nature and the occurrence of negative Eu anomaly (Eu/Eu* = 0.2). The latter distinct domains (Fig. 3a, b) it was necessary to use feature is intriguing because of the elevated Ba and Sr BSE imaging to locate the electron beam prior to contents, possibly inherited from magma mixing (see microprobe analysis; a BSE image for potassium below), and the low percent of feldspar in the granite, (Fig. 3c) illustrates the markedly varying content of thus inheritance due to mixing may be indicated. The that element. Representative matrix compositions are EREE is notably depleted compared to that in the San given in Table 3. The data (n = 59; Fig. 5) indicate Rafael and other Oligocene-Miocene granitoids of SE that there are large ranges in SiO2 (66 to 99 wt.%), Peru, although the chondritic patterns are similar A1203 (below detection (ND) to 19 wt.%), 1(20 (ND 752 D.J. KONTAK AND A. H. CLARK

and F and C1 were not detected (detection limit c. ~ CC-406 0.1 wt.%). The lack ofF is surprising in view of its 100 _ Field of peraluminous elevated content in biotite (see below) and the felsic Tertiary granites, SE nature of the matrix. Strong correlations are evident 0 ru between SiO2 and the major elements K, Na and AI (but note the convergence problem at high SiO2 values), although there is a notable scatter for K and o 10 Na in the least silicic areas. In contrast, Ca correlates relatively poorly with SiO2 compared to the former elements, in part due to contents approaching limits 0 of detection. o .% The matrix is strongly potassic with K20/Na20 CC-283 "~ ranging between 1.5 and 4.5 (Fig. 5); these values compare with the values of 1.55 and 1.86 for the whole-rock analyses (Table 2). In the K20+Na20 vs. SiO2 plot, the trend extends to a composition, about 65 wt.% SiO2 and 14 wt.% alkalis, approx- I I I I I I I I I I I I I I imating that of alkali feldspar, although the La Pr Sm Gd 137 Er Yb analyses do not conform to stoichiometric alkali Ce Nd Eu Tb Ho Tm Lu feldspar. Finally, we note that the most siliceous domains FIG. 4. Chondrite-normalized plot for granites of the (i.e. >92 wt.% SiO2) of the matrix contain substantial Cordillera de Carabaya region of SE Peru. The Minastira A1203 and alkalis. Detailed electron microprobing in granite is represented by CC-283, and the San Rafael such areas (10-15 Ixm apart) reveals a variation of granite by CC-406 (see text for discussion). The up to 3-4 wt.% SiO2; comparable variability was compositional range for the peraluminous Miocene- found for the relatively SiO2-poor areas. Pliocene granites (n = 15) in the study area is delimited (Kontak and Clark, 1988). Mineral chemistry

Sanidine has a uniform composition (Or67_76" Table 4 and Fig. 6), the maximum zonation detect- able in a single grain being 5-6 mole % Or. In some to 10 wt.%) and Na20 (ND to 3.5 wt.%), with minor grains an increase in Ba content from cores (e.g. to trace amounts of CaO, FeO and MgO. In addition, 0.21 wt.% BaO) to rims (0.48 wt.%) is noted. TiO2 is < 0.1 wt.%, BaO is generally < 0.1 wt.% but Cavities lined with anhedral-to-subhedral alkali in some areas attains 0.3 wt.%, P205 is ~< 0.3 wt.% feldspar grains have been observed in several sieve-

TABLE 3. Representative analyses of matrix material, Minastira granite, SE Peru

Point 3 -21 3-30 3 - 15 3 - 13 3-6 2-44 2-8 1-52 3 -4 1 2

SiO2 99.38 98.39 95.52 89.43 81.52 76.42 79.87 73.74 67.97 95.08 73.68 TiO 2 0.07 0.08 ND ND ND 0.09 ND ND ND 0.07 0.10 A1203 1.03 2.02 3.05 5.81 10.34 14.41 12.73 15.13 18.76 3.46 15.61 FeO 0.15 0.11 ND 0.11 0.17 0.08 ND 0.11 0.22 0.11 0.13 MnO ND ND ND ND ND ND 0.15 ND ND ND ND MgO ND 0.06 ND 0.14 ND ND ND ND ND 0.09 0.06 CaO ND ND ND 0.09 0.37 0.54 0.29 0.21 0.37 0.09 0.18 Na20 ND 0.22 0.39 0.94 1.94 2.53 2.41 2.37 2.81 0.39 2.36 I(20 ND 0.39 1.27 2.45 4.42 6.54 6.32 8.43 9.45 0.03 8.42 P20 s ND 0.13 0.23 ND ND 0.21 ND ND 0.26 ND 0.30 BaO ND ND ND ND ND ND ND 0.24 0.28 ND ND Total 100.63 101.40 100.46 98.97 98.76 100.82 101.77 100.23 100.12 100.22 100.84

Note: Points 1 and 2 refer to the BSE image in Figure 3. PERALUMINOUS GRANITE FROM PERU 753

2O 6 0 4 o ~ ~ tl* * ~1 ~ lO[ ~176149~ " o~.~,

,,, 176 O ~__ 0 J 0 6O 70 80 90 100 60 70 80 90 100 wt. % SiO 2 wt. % SiO 2

o4 o ~3 ~2 176 ~ , ~ ..

0 - 0 :: ":~ ' ' . ,, L:_ 60 70 80 90 100 60 70 80 90 100 wt. % SiO 2 wt. % SiO 2 25

o 10 ~ "~.'."., "'" "- ,o t 4 , r io. ~ ,o ,o o ; o ,; ,oo wt. % SiO 2 wt. % SiO 2

FIG. 5. Binary element variation diagrams for matrix material of the Minastira granite. As discussed in the text, the increase in apparent correlation towards higher silica values is a result of convergence. textured plagioclase grains and range in composition intergrown with or overgrown by the brown from OrssAb4oAn2 to Or65Ab35 (Fig. 6); thus they are ferromagnesian phase have compositions of more sodic than the microphenoerystic sanidine. A An35_55; (4) the matrix plagioclase, An33_4o , is not single grain of alkali feldspar included in biotite has a the most sodic in composition; and (5) micropheno- composition of OrssAb15, with 1.32 wt.% BaO. trysts range from An2o to Arts6 with an apparent gap Plagioclase exhibits a large range in bulk at An4o-s0. Zoning profiles for representative grains composition (Table 4; Fig. 6, 7) with limiting (Fig. 8) indicate that at least in some cases the sieve- compositions of AnssAbasOr4 and An2oAb75Or5; we textured plagioclase is more calcic than clear areas. emphasize the high potassium contents (el. Deer et Biotite (Table 5; Fig. 9) is annitic in composition, al., 1966). The compositions of the rims of more than 90% of the samples falling in the plagioclase grains are consistently in the range compositional range Fe/(Fe+Mg) = 0.45 Jr 0.05. An27 to Anal , and oscillatory zoning of some 4-5 MgO varies from 10.5 to 14.8 wt.%, TiO2 from 2.5 to mole % An occurs. An analysis (An]5) that diverges 4.6wt.% and A1203 from 15 to 18.7 wt.% from this trend was for a grain adjacent to a cavity (Fig. 9,b,c). Many of the grains incorporate abun- infilled by quartz and sanidine. Subdivision of the dant, free-grained, opaque domains, but no composi- compositional data in terms of the plagioclase tional differences were detected in microprobe morphotypes (Table 4; Fig. 7) reveals no systematic analyses of these areas vs. clear areas within biotite. trends, but shows that: (1) overgrowths on the brown A single analysis of a biotite separate (Table 2) ferromagnesian phase are An3o-as; (2) sieve-textured compares favourably with the electron microprobe rims of phenocrysts are An2s-44; (3) patches data, except for lower MgO and K20 contents which 754 D. J. KONTAK AND A. H. CLARK

TABLE 4. Representative analyses of feldspars, Minastira granite, SE Peru

Phase K-feldspar K-feldspar K-feldspar Plagioclase Plagioclase Plagioclase Plagioclase Point 1-3 1-4 2-38 1-8M 1-9C 1-14C 1-60C

SiO2 66.95 67.35 65.87 62.22 62.32 58.54 56.00 AlzO3 19.04 19.00 18.58 24.59 24.57 27.08 28.75 CaO 0.25 0.07 0.11 6.26 6.31 9.28 11.34 Na20 3.44 2.79 2.45 6.79 7.05 5.59 4.85 K20 11.13 10.97 12.12 0.79 0.81 0.46 0.35 BaO 0.48 0.21 0.26 ND ND ND ND Total 101.29 100.59 99.39 100.65 101.65 100.95 101.29

An 1.6 0.0 2.0 32.1 31.3 46.6 55.2 Ab 31.0 29.7 23.2 62.6 63.5 50.8 42.2 Or 67.2 70.2 75.8 5.2 5.0 2.5 2.4

Phase Feldspar Feldspar Plagioclase Plagioclase Plagioclase Plagioclase Plagioclase Point 3-231 3-28 3-41C 3-42R 3-62Ralt 3-69Ralt 3-63Calt

SiO2 67.16 67.19 62.71 59.88 58.82 60.95 60.54 A1203 20.56 19.73 24.41 26.18 26.99 25.29 25.83 CaO 1.78 1.32 6.13 7.81 9.03 6.88 7.49 Na20 5.02 3.48 6.44 5.85 5.39 6.12 6.09 K20 5.19 6.12 0.91 0.54 0.43 0.72 0.66 BaO ND 0.29 ND ND 0.13 ND 0.25 Total 99.71 98.13 100.60 100.26 100.79 99.96 100.86

An 10.9 9.0 32.4 40.7 46.4 36.6 38.5 Ab 52.9 42.2 62.1 55.7 50.8 58.9 56.9 Or 36.2 48.8 5.5 4.6 2.8 4.5 4.6

Note: M = matrix; R = rim, C = core; alt = alteration; I = inclusion in K-feldspar

may reflect limited chloritization. The Fe203/FeO biotite enclosed by quartz is the most Mg-rich, but ratio is very low (0.06), consistent with similarly low contains negligible F (Fig. 9d). ratios for the whole-rocks and other biotites from SE The trace element chemistry of the biotite separate Peruvian granites (Kontak, 1985; Kontak et al., 1984; is given in Table 2. Relative to micas in other Kontak and Clark, 1988); in the FeZ+-Fe3+-Mg peraluminous granites in this area, the Minastira diagram of Ishihara (1977) the data plot along the sample is enriched in Ba and depleted in Rb. The QFM buffer. enrichment in Ba is in conformity with its more The biotite is distinguished from biotites in magnesian composition (Fig. 9a) and is compatible peraluminous granites in the region by its high with some Ba enrichment in the feldspars noted above. A1203 and MgO contents (Fig. 9). Analytical data, Apatite (Table 6) is F-rich (1.7 to 2.8 wt.%), with summarized in Fig. 9, show that although there is a <0.5 wt.% C1, minor Fe (0.5 to 1.5 wt.% FeO), and general trend of increasing fluorine with decreasing traces of Mn (to 0.4 wt.% MnO) and Na (to Fe/(Fe+Mg), exemplifying the Fe-avoidance prin- 0.25 wt.% NazO). No zonation has been detected ciple (Bailey, 1984), there is overall a large variation within grains and there is no compositional variation in F content [below the limit of detection (c. that can be related to morphology or to the presence 0.1 wt.%) to 4.5%]. This diagram also illustrates of inclusions. An analysis of an apatite included the intra-grain variation in both major element [e.g. within a coarser grain of this mineral revealed no Fe/(Fe+Mg)] and F contents. In contrast to the wide difference in composition. range observed for F, the C1 contents are uniformly Electron microprobe data for the brown to greenish- tow at c. 0.15 wt.% (Table 5). It is of interest that a brown ferromagnesian phase are summarized in the PERALUMINOUS GRANITE FROM PERU 755

An I

I core E~ rim //~ single grains (microphenoerysts) r////~r.,cd

matrix phase

coring alteration plagioclase phase (n - S0) mantling alteration phase [ 8 sanidine intergrown with quartz in sieve-textured coarse sanidine grains (n ffi8) plagioclase (o ffi6)~. ~4i area rich in melt inclusions 1: 20 30 40 50 6O Ab Or mole % An Fro. 6. Ternary plot for feldspar compositional data from FIG. 7. Histograms summarizing the compositional data the Minastira granite. Not included in this diagram are (in mole % An) for different textural varieties of compositional data from probe traverses of plagioelase plagioclase discussed in the text. Significantly the shown in Fig. 7. matrix plagioclase grains are generally more ealcic than isolated microphenocrysts.

form of binary element plots for the major oxides, which include SiO2, FeO, MgO and A1203 (Fig. 10); representative analyses are given in Table 7. The data peraluminous felsic suites, both intrusive and have been subdivided on the basis of mode of extrusive, are plotted for comparative purposes and occurrence, namely: (1) within minerals such as there is generally a good correlation, the exception plagioclase, biotite and apatite, (2) as small euhedral being for Mg-enriched cordierites from andesitic grains in the matrix, and (3) as large grains of zoned volcanics in Martinique (Maury et al., 1985). From nature (i.e. with different shades of green and brown). this comparison plot we can infer that the Fe-depleted The data reveal a broad range in chemistry with both FeO and MgO varying by 10-12 wt.%, SiO2 by about 20% and AI203 by 10%. The totals axe low, 83 to Q zone of melt inclusions i 92 wt.%, indicating the presence of water, given that ~ 40 ~_._.~re o._fpl~oclase eo ' neither F nor C1 was detected in microprobe analysis. With respect to the nature of this material the . 20FOo *o * *. following points are germane: (1) most analyses reveal an FeO:MgO ratio of 1:1 to 1:4 despite the ] large range in the absolute abundances of these (~) core ofplagioclase elements. However, the FeO:MgO ratio increases at "~ 40 " % ~. elevated content values of MgO with a marked break l e at 7 wt.% MgO; (2) in most types of occurrence, the o 20 .~ I zoneof meltinclusions 50 pm FeO:MgO ratio is consistent, although with some exceptions (e.g., matrix grains) the absolute abun- dance of these elements varies. Thus, a chemical zonation which is consistent with colour variations is ~ J presumed to be present within grains; and (3) there is a I systematic decrease of the ferromagnesian elements 20 zone of meR mch.mi~ with increasing SiO2, whereas A1203 remains fairly constant. Thus, the dominant elemental exchange is F[o. 8. Partial core-rim profiles of sieve-textured (Fe + Mg) ~- Si. plagioclase grains from the Minastira granite. Note that Identification of the ferromagnesian phase remains the distance to the grain margins in profiles 8b and 8c is uncertain, but a tentative interpretation is offered. In greater than indicated with the scale. The indicated scale Fig. 10 cordierite analyses from a variety of of 50 lam applies to all three profiles. 756 D. J. KONTAK AND A. H. CLARK

TABLE 5. Representative analyses of biotite, Minastira granite, SE Peru

Grain 1 1 2 2 3 3 4 5 6 6 Point 3-3 3-32 3-39 3-40 3-44 3-45 3-67 3-77 3-65 3-66

SiO2 37.46 37.48 33.92 36.11 36.77 36.70 36.77 36.10 35.87 31.82 TiOz 3.75 3.99 3.12 3.42 3.17 3.13 3.01 3.82 3.28 2.77 A1203 15.21 15.39 15.75 16.69 16.36 16.76 16.86 15.33 16.89 19.94 FeO 16.47 14.70 24.30 19.03 18.48 17.98 18.52 14.81 16.74 21.46 MnO ND 0.28 0.09 0.12 ND 0.12 0.11 0.19 ND ND MgO 14.04 14.78 11.09 12.23 12.08 12.00 12.14 14.77 13.41 12.23 CaO 0.00 0.00 0.00 0.00 0.00 0.06 0.00 0.00 0.00 0.00 Na20 0.64 0.77 0.57 0.64 0.61 0.61 0.62 0.69 0.56 0.80 K20 8.89 8.94 8.50 9.33 9.16 8.96 9.19 9.24 9.19 7.86 BaO 0.64 0.52 0.00 0.00 0.39 0.47 0.51 0.00 0.00 0.20 F 4.35 4.34 1.84 2.80 0.58 0.13 1.10 0.00 3.96 3.16 CI 0.14 0.15 0.13 0.15 0.15 0.12 0.18 0.I1 0.12 0.15 Total 1.59 101.34 99.31 100.52 97.75 97.04 99.63 96.06 100.02 100.39 F, CI=O 1.86 1.86 0.79 1.20 0.27 0.07 0.50 0.02 1.69 1.36 Total 99.73 99.48 98.52 99.32 97.48 96.97 99.13 96.04 98.33 99.03

Structural Formulae (11 Oxygens) Si 2.772 2.761 2.596 2.684 2.739 2.728 2.728 2.750 2.684 2.409 Al(iv) 1.227 1.239 1.403 1.316 1.261 1.272 1.272 1.250 1.316 1.591 Al(vi) 0.093 0.092 0.016 0.147 0.180 0.202 1.474 0.092 0.169 0.191 Ti 0.209 0.220 0.176 0.187 0.176 0.176 0.165 0.200 0.187 0.154 Fe 1.023 0.902 1.562 1.188 1.155 1.122 1.144 0.913 1.045 1.364 Mn 0.000 0.022 0.011 0.011 0.000 0.011 0.011 0.011 0.000 0.000 Mg 1.551 1.617 1.265 1.353 1.342 1.331 1.342 1.628 1.960 1.320 Ca 0.000 0.000 0.000 0.000 0.022 0.011 0.000 0.000 0.000 0.000 Na 0.088 0.110 0.088 0.088 0.088 0.088 0.088 0.099 0.077 0.121 K 0.836 0.836 0.836 0.880 0.869 0.847 0.819 0.869 0.880 0.759 Ba 0.000 0.011 0.000 0.000 0.011 0.011 0.011 0.000 0.000 0.011 F 1.012 1.067 0.451 0.660 0.143 0.033 0.253 0.000 0.935 0.759 C1 0.022 0.022 0.022 0.022 0.022 0.011 0.022 0.011 0.011 0.022

samples with FeO:MgO of 1:2 to 1:4 may represent propose from this information that the ferromagne- loss of Fe during alteration or represent another sian phase may represent altered cordierite. However, distinct phase. In Fig. 11, a ternary plot (wt. %) of if one assumes that the variety of textural relation- SiO2-A1203-(FeO+MgO), the ferromagnesian phase ships all represent the same original phase rather than is compared to compositions of a variety of potential a variety of phases, then several generations of primary and secondary minerals, including cordierite, cordierite are required, some of which have not been osumulite, pinnite and vermiculite. In addition, we observed in either igneous or metamorphic rocks note that the range of analyses of the unknown based on our review of the literature. An alternative phase(s) indicates a chemistry comparable to scenario, but one which we do not favour because of smectite-, vermiculite- or osumulite-like minerals the added complexity, is that at least two different based on the MinIdent program (D. Smith, pers. precursor mineral phases existed, one being commun., 1990). Thus, alteration (i.e. hydration) of a cordierite and the second an unknown. primary phase such as cordierite or osumulite to a mixed assemblage of secondary clay-type minerals Discussion can account for the observed chemistry. In this regard it is interesting to note that Haslam (1983) reported The processes of magma mixing and commingling an isotropic alteration product after cordierite with a are generally well accepted in igneous petrology (e.g. composition not unlike that noted herein (see Figs. 10 Walker and Skelhorn, 1966; Anderson, 1976; Wiebe, and 11), albeit slightly Mg-deficient. Thus, we 1973, 1992; Vernon, 1984, 1990) and they have been PERALUMINOUS GRANITE FROM PERU 757

wt.%AlzO 3 San Rafael (~) wt. % TIO 2 | m 5.0 field 20 -- ~"~ ~.._.. Mlnastir, ooo~ % 3.0 | ~

16 4- [ [ I~1 Lo I I I 8 10 12 14 0.35 0.40 0.45 0.50 0.55 wt. % MgO Fe/(Fe+Mg)

4.0 0 1.4 + Q 2.0 biotite in 0 ~ ~ V 4.

I I I w' I 0.35 0.40 0.45 0.50 0.55 0.35 0.40 0.45 0.50 0.55 Fe/(Fe+Mg) Fe/(Fe+Mg)

FIG. 9. Binary element diagrams summarizing the chemistry of biotite from the Minastira granite. (a) A1203 vs. MgO with bulk analyses for biotite separates for Miocene granites of SE Peru (Kontak, 1985) compared to the field for the Minastira granite. Also shown is the field of biotite composition for the San Rafael granite as determined from EMPA (unpubl. data of Kontak). (b) ivA1 vs. Fe/(Fe+Mg); note that sample with extreme Fe-enrichment is in part chloritized and thus the Fe-enrichment may reflect secondary processes. (c) TiO2 vs. Fe/(Fe+Mg). (d) F vs. Fe/ Fe+Mg) for biotite grains with the analyses grouped according to individual grains (i.e. same symbol). Note the larger intra-grain variation with respect to both variables plotted. documented in widely divergent settings (e.g. Castro 1992). In fact, Castro et al. (1991), in recognizing the et al. 1990; Feeley and Grunder, 1991; Seaman and widespread occurrence of mixing and hybridiization Ramsey, 1992; Blake et al., 1992; Stimac and Pearce, within mafic-felsic systems, proposed a classifica-

TABLE 6. Representative analyses of apatite, Minastira granite, SE Peru

Point 3-35 3-36 3-53 3-54 3-55 3-73 3-74

SiO2 0.25 0.22 ND ND ND ND ND FeO 1.56 1.53 0.57 0.8 0.51 0.58 0.97 MnO 0.42 0.35 0.18 0.24 0.2 0.3 0.35 MgO 0.32 0.35 0.29 0.28 0.19 0.18 0.15 CaO 52.62 52.67 53.93 53.33 53.52 53.58 53.85 Na20 ND ND 0.14 0.25 0.18 0.15 0.09 P205 42.29 42.12 42.31 42.26 42.4 42.2 42.44 CI 0.48 0.52 0.32 0.4 0.28 0.29 0.27 F 2.27 2.02 2.79 2.43 2.45 2.29 2.4 Total 100.21 99.78 100.53 99.99 99.73 99.57 100.52 F, CI=O 1.06 0.97 1.25 1.11 1.09 1.03 1.07 Total 99.15 98.81 99.28 98.88 98.64 98.54 99.45 758 D.J. KONTAK AND A. H. CLARK

-- "~ inclusion in apatitOe (16) 12 -- biotite host 0 0 -- small matrix grains 10 Z~ -- in plagioclase- 0 -- core to riXn~S ~ 8 -- all others are _ coarse grai O0 ,~ 6 .8 1:4 2 0 u" I I I I I I I I I (3 0 igneous eordierite eq m 60 @@ ~" 149 CI isotropiephasealteration ot~l r~

- 0 0 I I I I I I I I I 0 o~ od, eooO db 3O " _o m

- I I I I I I I I I 2 4 6 8 10 12 wt. % MgO

FIG. 10. Binary element diagrams for the brown and greenish-brown ferromagnesian phase in the Minastira granite with data also plotted for igneous cordierites from peraluminous intrusive and extrusive suites (Wybom et al., 1981; Allen and Ban', 1983; Maillet and Clarke, 1985; Maury et al., 1985). The yellow isotropic phase, an alteration product after cordierite, is from Haslam (1983). Note that the most magnesian cordierites are from an andesite (Maury et al., 1985) and thus reflect the magnesium-rich nature of the host. tion scheme which takes into account the dominant the petrology to develop a genetic model for the process and end-member in such suites. In the origin of the marked variability of textural and following section we expand on some aspects of chemical features within these rocks. PERALUMINOUS GRANITE FROM PERU 759

TABLE 7. Representative analyses of unknown phase, Minastira granite, SE Peru

Point 1-45 1-47 1-62 2-33 2-46 4-11 light brown dark brown plag interior plag interior plag rim apatite host

SiO2 50.53 48.42 43.03 53.33 54.75 41.54 TiO2 ND ND ND ND ND ND A1203 25.87 21.48 25.75 23.06 23.82 23.98 FeO 6.73 10.10 10.61 1.43 1.72 14..04 MgO 7.60 11.50 7.33 6.08 6.24 8.86 CaO 0.20 0.31 0.70 0.23 0.33 1.47 Na20 0.44 0.58 0.27 0.22 0.21 0.27 K20 0.33 0.32 0.30 0.28 0.29 0.16 MnO ND ND ND 0.12 0.09 0.21 BaO ND ND ND 0.16 ND ND Total 91.70 92.71 87.99 84.91 87.45 90.53

Point 3-26 1-7 1-53 1-55 1-56 4-117 plag rim matrix grain light brown light brown light brown biotite host

SiO2 43.22 40.89 49.23 47.58 45.95 43.93 TiO2 0.18 ND ND ND ND ND A1203 22.76 19.16 28.34 28.19 28.34 23.65 FeO 12.96 15.95 5.51 4.47 7.02 3.72 MgO 8.31 11.63 5.90 5.64 8.63 9.68 CaO ND ND ND ND ND ND Na20 0.82 0.26 0.56 0.53 0.27 0.48 K20 0.22 0.25 0.50 0.29 0.35 1.09 MnO 0.26 0.24 0.38 0.29 0.11 0.18 BaO 0.09 0.26 ND ND 0.10 ND Total 88.82 88.64 90.42 86.99 90.77 82.73

Evidence for magma mixing. The occurrence of Chemical features of plagioclase that indicate mixing magma mixing at Minastira can be inferred from the are compositional reversals, polymodal compositions mineralogy and whole-rock and mineral chemistry. and spikes in the zoning profiles (cf. Gourgaud and Several textural features present in the Minastira Villemant, 1992; Gourgaud et al., 1989; Vogel et al., rocks are frequently cited as being suggestive, if not 1987; Castro et al., 1990). diagnostic, of magma mixing, viz. sieved plagioclase The bulk chemistry of the samples examined, in (MacDonald and Katsura, 1965; Eichelberger, 1975, particular the A/CNK values and the ehondritic REE 1978; Kawamoto, 1992), resorption features such as pattern, contrast with values and patterns typical of embayed quartz (Smith and Carmichael, 1968; similar age granitic rocks in the area (e.g. San Rafael Sakuyama, 1978), internally fractured quartz (Kontak granite; Kontak and Clark, 1988). The highly et aL, 1986), pyroxene coronas mantling quartz peraluminous nature of the samples is anomalous (Eichelberger, 1978; Stimac and Pearee, 1992), melt and presumably reflects the abundance of the brown inclusions of mixed chemistry (Anderson, 1976) and ferromagnesian phase (cordierite ?), whereas EREE disequilibrium or mixed mineral associations is notably depressed compared to rocks of similar (Eichelberger, 1975; Sakuyama, 1978; Gerlach and SiO2 content in the area (Fig. 7) and is similar to that Groves, 1982; Gourgraud et al., 1989; O'Brien et al., typical of intermediate volcanic rocks such as those 1988). We also infer that the internal features of the of the Central Andes (e.g. Dostal et al., 1977). The biotite suggest incipient melting by analogy with biotite chemistry is also unusual, the elevated MgO other magmatie suites (e.g. Nixon, 1988; Stimac and contents suggest equilibration with a relatively marie Pearee, 1992) and direct comparison with experi- melt. Nonetheless, the elevated F content of the mental work of biotite stability in felsic systems (e.g., biotite (to c. 4 wt.%) is more typical of highly Clemens and Wall, 1981); therefore, biotite textures evolved and volatile-rich felsic suites (Bailey, 1984). also support magma mixing and commingling. The heterogeneous distribution of F in biotite, which 760 D. J. KONTAK AND A. H. CLARK

SiO 2 66 wt.% SiO2 to nearly pure SiO2, with the siliceous end member containing significant amounts of alkalis and A1203; and (8) minor amounts of plagioclase with concentrically-arrangedinclusions of sillimanite and biotite and micro-inclusion rich apatite. On the

ferromaglellsn basis of the aforementioned features the following petrological model is proposed. Generation of at least two diverse melts occurred at depth with subsequent mixing of the two as a result of favourable melt densities and temperature (e.g. Sparks and Marshall, 1986). From the nature of the crystal assemblage and the matrix we can identify one end member as being felsic, probably rhyolitic in nature. The presence of concentrically-arranged sillimanite AlzO3 FeO+MgO needles and rare biotite within plagioclase cores, rare sillimanite as constituents of plagioclase fragments, FIG. 11. Ternary plot (wt.%) for SiO2-A1203-(FeO + and coarse micro-inclusion-rich apatite suggest that MgO) showing the compositional range of the brown to such felsic melts may have originated from crustal greenish-brown alteration phase in relation to composi- fusion, perhaps similar to the slightly younger tions of cordieritc (as in Fig. 10), a yellow isotropie Miocene-age, peraluminous Macnsani volcanics just phase after cordierite (see text; Haslam, 1983), osumu- to the north where similar textures are observed lite (Piehavant et al., 1988a) and vermiculite and pinnite (Pichavant et al., 1988a, b). Assuming that cordierite (from Deer et al., 1966). Note that analyses of the brown was indeed present in the melt, then this phase is ferromagnesian phase observed in the Minastira granite inferred to be early magmatic based on the frequent plot in a field defined by the phases mentioned. euhedral shapes and occurrence as inclusions in quartz and feldspars. In this regard the cordierite conforms to Clarke's (1995) Type 2(e) eotectic cordierite. Hercinitic spinel is also interpreted as an early phase is not unlike that noted by Pichavant et al. (1988a) based on its occurrence, i.e. a loose spatial relationship for biotites in the Macusani volcanics, is matched by to the cordierite phase and as inclusions within the wide range of plagioclase compositions and plagioclase. Thus, generation of the felsic melt is textures and the local enrichment of this mineral in restricted to upper crustal levels (i.e., ~<4-5 kbar) and BaO (to 1.3 wt.%). maximum temperatures ofc. 800-900~ based on the Despite the non-exposure of cuspate interfaces presence of the cordierite phase, hercinitic spinel and between relatively marie and felsic end members (see F-rich nature of some of the biotite (Clemens and discussion below) that is common in the subvolcanic Wall, 1981; Pichavant et al., 1988a and references environmentwhere magma commingling occurs (e.g. therein). The compositions of inclusion free, euhedral Castro et al., 1990; De'Lemos, 1992; Wiebe, 1973, to subhedral plagioclase of clear magmatic origin is 1979, 1992; Vernon, 1983), the aforementioned consistent with a hasahic andesitic composition for the features are clearly sufficient to indicate the marie melt. The absence of ubiquitous pyroxene occurrence of magma mixing at Minastira. The coronas manthng quartz, as seen in the shoshonites nature and origin of the end member components at nearby Moromoroni (Kontak et al., 1986), implies remains problematic, however. that the marie component was subordinate to the felsic Model for genesis of the subvolcanic Minastira melt. The textures within biotite that suggest incipient granite. Several features that are of particular breakdown would be consistent with superheating of importance in terms of understanding the origin of the felsic melt and its phenocrystic assemblage due to the Minastira granite include: (1) fracturing or mixing with resultant magma temperatures close to or breakage of minerals, plausibly due to rupturing of exceeding that required for biotite breakdown (i.e. melt inclusions during decompression of the magma; 900 ~ to 950~ Clemens and Wall, 1981). (2) internal fracturing of quartz that suggests thermal Mixing of the two melts must have occurred prior shock; (3) internal melting of biotite (e.g. Stimac and to emplacement at the present level for the following Pearce, 1992; Nixon, 1988); (4) internal melting of reasons: (1) rapid crystallization is implied based on both alkali and plagioclase feldspar; (5) hetero- the grain size and textures within the matrix (Fig. 3); geneous mineral chemistry as noted, for example, in (2) the absence of enclaves or globules of marie biotite; (6) the abundance of a ferrromagnesian phase composition; (3) inclusions of glomeroclasts that may represent precursor cordierite; (7) a containing intergrown sieve-textured plagioclase; quenched matrix ranging in composition from c. (4) complex textural relationship between plagioclase PERALUMINOUS GRANITE FROM PERU 76l and the brown ferromagnesian phase with both the strongly peraluminous cordierite + sillimanite phases mantling each other. Although not indicative granites such as occur at nearby San Rafael. In this of relative timing or location of mixing, the context, the intimate interplay of mafic-felsic abundance of internal fractures within quartz may magmatism documented here is similar to that be a result of thermal shock. The generally fractured documented throughout much of the geological and broken nature of crystals may have originated record for both intrusive and extrusive igneous rocks during convection (i.e. turbulent) within a magma (Anderson, 1976; Castro et al., 1991). chamber or during passage through a conduit, the latter fitting in with the mechanism suggested by Conclusions Blake and Campbell (1986) for magma mixing. However, an alterative mechanism for fracturing of The Minastira granite, a subvolcanic plug of fine- phenocrysts recently discussed by Best and grained granitic rock, has preserved textures Christiansen (1997) relates rupturing of crystals to indicative of a history involving mixing of at least vesiculation of melt that was trapped at higher two magmas, a volumetrically dominant felsic pressures as inclusions within phenocrysts. Thus, component, which is in part restitie, and a less crystallization of some of the phenocrystic compo- voluminous marie one. The felsic melt is represented nent at a different level is suggested and residence by variably fractured, altered and embayed crystals within the area of mixing, most probably within the of quartz, feldspar and biotite, with minor coarse- upper crust, must have occurred. The extent of grained and melt- and fluid-inclusion rich apatite, residence time has been estimated by various workers and possible cordierite, whereas the marie melt is based on diffusivities in Fe-Mg minerals (e.g. represented by calcic plagioclase and rare pyroxene Gerlach and Groves, 1982) or dissolution rates in mantles on quartz. The process of magma mixing is development of sieve-textured plagioclase based on reflected in the ubiquitous presence of sieve-textured experimental (Lofgren and Norris, 1981) and plagioctase with complex textural relationships. The theoretical (Andersson and Eklund, 1994) work. absence of marie enclaves indicates that physico- Intervals calculated are of the order of a few hours chemical conditions of the mixing were conducive to to hundreds of hours and suggest relatively rapid homogenization (i.e. chemical diffusion) and an post-mixing eruption where volcanic rocks have been apparently homogeneous rock is now observed. The the focus of study. The general absence of sodic-rich combination of glomeroclasts of crystals that mantles on plagioclase despite the occurrence of such originated from both the mafic and felsic end compositions in the matrix are taken to indicate that members, and a quenched quartz-feldspar matrix quenching occurred before reaction between crystals indicate that the mixing occurred in an underlying and melt could take place. Final emplacement of the magma chamber and that the Minastira granite may mixed melt, within a narrow conduit at shallow have been a conduit or plug. levels, resulted in quenching of the magma and preservation of the disequilibrium assemblage. Acknowledgements Implications for genesis of granites of the Inner Arc region of SE Peru. As discussed elsewhere This research was conducted while the senior author (Kontak et al., 1984, 1986; Kontak and Clark, 1988), was a graduate student at Queen's University and the close spatial and temporal association of involved in the Central Andean Metallogenetie shoshonitic magmatism and peraluminous granites Project (1979-1984). Field work in Peru and of latest Oligocene age in the Cordillera Oriental laboratory studies supported by the Natural region of SE Peru suggests a cause and effect Sciences and Engineering Research Council of relationship. Although the marie component of the Canada (grant to A.H. Clark). The field work was Minastira system cannot be identified unambigu- carried out with the logistical assistance of Minsur ously, the nearby c. 25 Ma Moromoroni shoshonites S.A., through the good offices ofF. Zavaleta. Mr. B. are of appropriate age, as are other centres of mafic McKay, Dalhousie University, is acknowledged for magmatism in this part of the Cordillera Oriental providing assistance with the electron microprobe (Sandeman, 1995; Sandeman et al., 1995), and work. The authors appreciate the comments of indicate that at this time mantle-derived magmas journal reviewers that resulted in clarification and ascended to shallow crustal levels and prompted the improvement of the text. advection of heat and volatiles into upper crustal levels. The latter phenomenon would have promoted References the melting of parts of the thick metasedimentary sequence that characterizes the Inner Arc region of Alien, P.L. and Barr, S.M. (1983) The Ellison the Central Andes, which is dominated by pelitic pluton: a cordierite-bearing monzogranitic intrusive sequences (Laubaeher, 1978), and thus given rise to body in southwestern Nova Scotia. Canad. Mineral., 762 D.J. KONTAK AND A. H. CLARK

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